A method of making a golf ball comprising the steps of providing a preform comprising an uncured polybutadiene composition; coating the preform with a cure-altering material comprising an organosulfur compound, an inorganic sulfide compound, or a metal-containing organosulfur compound; curing the coated preform at a predetermined temperature to form a crosslinked golf ball core having an outer surface having a first hardness and a geometric center having a second hardness greater than the first to define a negative hardness gradient; and forming a cover layer about the core to form the golf ball.